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Mass spectrometric studies of a vacuum system fitted with both getter-ion and sublimation pumps
Mass spectrometric studies of a vacuum system fitted with both getter-ion and sublimation pumps S W Newton and J O'Neill, Ferranti Ltd, Wythenshawe, Manchester
The operation of a titanium sublimation pump with various types of filament is described. The partial pressures of residual gases in a stainless steel system were investigated during pump-down and also at the base pressures reached. The main residual gas after getter-ion pumping was hydrogen. The use of a sublimation pump in parallel with the getter-ion pump reduced the pressures of all gases, the main reduction being with hydrogen. A sublimation pump used alone did not pump the inert gases, but rapid pumping of all gases present was observed when both pumps were used together. The composition of residual gases after the pressure had fallen to 10-s torr during a normal pump-down was not greatly affected by the gas used for filling. At this pressure the composition was determined mainly by the gases evolved from the walls of the system. 1. Introduction Sublimation pumps are being increasingly used in conjunction with getter-ion pumps for the production of ultra-high vacua, and for the rapid evacuation of large volumes. They have a high pumping speed, and are simple to use. Their operation depends on the gettering action of a freshly deposited layer of titanium, and no ionising voltages nor magnetic fields are applied. The following study has been made by mass spectrometer of the residual gases in a system with both kinds of pump. 2. Description of system The system was constructed as in Figure 1, and consisted of a stainless steel cylinder of average diameter 6 inches and 50 inches long. It was fitted with a nude ion gauge (Bayard-Alpert type), Pirani gauge, viewing port and mass spectrometer (AEI SUBLIMATION PUMP TERMINALS
SUBLIMATION PUMP
CTROMETER NUDE ION GAI GAUGE NE--
VIEWING PORT
--~.~
..... rZ~_ 3
~
'~--'~:'VALVES
VOLUME OF SYSTEM 25 LITRES MATERIAL STAINLESSSTEEL
Figure 1. Vacuum system with sublimation pump
type MS10). At one end was a getter-ion pump (Ferranti FJD80) and at the other end was a sublimation pump (Ferranti FED10). Water cooling was applied to the regions surrounding the sublimation pump and the ionisation gauge. Before the series of measurements described in this paper the pumps had been operated whilst the whole system was baked at 200°C for three days. 3. S u b l i m a t i o n p u m p The sublimation pump had three filaments of an alloy containing 85 per cent titanium and 15 per cent molybdenum, 2.1 mm in diameter and 130 mm long, as described by McCracken 2. The filaments were used sequentially, being heated by ac to a temperature sufficient to evaporate the titanium on to the adjacent walls of the system, thereby providing the pumping action. Typical lives of the filaments for continuous operation ranged from 7 hours at 55 amps operating current to 15 hours at 45 amps. However, the filaments were normally cycled when the vacuum was better than 10-6 tort, so giving an increased overall life. A 25 per cent cycle, 1 minute "on", 3 minutes "off", was used down to 10- s torr, and 10 per cent or less when the pressure was still lower. Constant current operation was used. The voltage to be applied increased during life by about 60 per cent, so giving an indication of the remaining life of the filament. Pumping action was obtained throughout life, until bu'.T~-out occurred. At a running current of 55 amps, the average rate of deposition of titanium was approximately 3.4 micrograms per sq cm per minute. Figure 2 shows the appearance of a pump with filaments after completion of life test. The straightest filament is of the Ti/Mo alloy, and shows a marked crystal growth with offsetting of the crystals. Several grooves are also to be seen running circumferentiaUy round the wire at the crystal boundaries, indicating possibly some form of chemical attack by gas at these positions. At an earlier stage of life, the filament was bowed in a similar manner to the other filaments shown, but it had pulled straight again before burn-out occurred. 40 per cent of the available titanium had been used during life. The other two filaments consisted of a core of three strands of titanium wire, with tungsten overwinding. The alloy filament has lost
Vacuum/volume 16/number 12. Pergamon Press LtdlPrinted in Great Britain
677
S W Newton and d O'Neill: Mass spectrometric studies of a vacuum system 12 II
IO 9 8
(o)
7 6 5
(])GETTER ION PUMP
RELATIVE4 A~UNDAN~ I
O
2
:,1 I
12 1~I 16 18
12 202'22426 30323'43038404244 m/e
Figure 2. Sublimation pump filaments after completion of life test titanium uniformly from the entire length, but the other type has lost material only from the region close to the position of burn-out. This results in a short life and inefficient pumping due to the restricted area of the film of condensed titanium. A third type of filament has been used in a few tests only, having a tungsten core with two overwinding wires of titanium and one of molybdenum. This type did not always burn out when the titanium had been completely evaporated, so continuity of the filament is not a positive indication of satisfactory pumping action in a system without vacuum gauges. 4. R e s i d u a l gases after p u m p - d o w n
The mass spectra obtained on different days under similar conditions did not all show identical results, but the general trend was always maintained. In the first experiment, the system was evacuated to 10-8 tort, the ion pump switched off and dry nitrogen from boiledoff liquid nitrogen, admitted to a pressure of 5.2 x l0 -3 torr. The ion pump was now operated until the pressure fell to 5 x 10- s torr. Figure 3 shows the spectrum obtained on the MS10. Hydrogen (mass 2) showed the most prominent line, with water (masses 17 and 18) and methane (masses 12-13-1415-16) also prominent. Smaller amounts of nitrogen and carbon monoxide (mass 28), argon (mass 40) and carbon dioxide (mass 44) were present. The sublimation pump was now operated until the pressure was reduced to 5 × 10-9 tort. The most noticeable change in the mass spectrum (Figure 3b), was the reduction in hydrogen content from 12.3 units to 0.8 units, but all lines were somewhat reduced. The effect of using a sublimation pump without a getter-ion pump is shown in Figure 4a. Normally a sublimation pump is used in parallel with an ion pump. Nitrogen was admitted to a pressure of 5 × 10-3 ton'. On operating the sublimation pump, the pressure dropped rapidly to 1.6 × 10-s ton. within 1 minute, and then fell slowly to 6 × 10-6 torr in 45 minutes. The main residual gas now was the rare gas argon, the calculated line height being 1600 units, with methane, nitrogen, neon, carbon dioxide and ethane also present. Hydrogen and water vapour were only present in very small quantities. A similar test, in which the system was filled with dry air at atmospheric pressure instead of nitrogen gave a very similar spectrum, except that the heights of the neon lines were considerably increased. In both cases the amount of oxygen present was below the sensitivity limit of the mass spectrometer, 0.02 units, corresponding to a pressure of 0.76 × 10-*o ton'. 678
(b)
~
-,~ GETTER ION PUMP WITH SUBLIMATIONPUMP
t I
,Ill
,
2 ,2,4,6,8
..,
. . . . .
I
.,
2 0 2 2 24 2 6 2 s 3 0 3 2 3 4 3 6 , 3 s 4 0 , 2
m/¢
44
Figure 3. Residual gases after pump-down:
(a) After getter-ion pumping (b) After getter-ion pumping with sublimation pumping The getter-ion pump was now operated until the base pressure of 1.8 × 10-9 ton. was reached as measured on the ion gauge. The most marked changes in the mass spectrum occurred with the rare gases, argon and neon. These pressures fell to less than 0.01 per cent of the value with sublimation pump alone, but significant reductions were noticed with all gases. Hydrogen and water vapour had already been pumped by the sublimation pump, for the pressures in these cases were only reduced to 7 per cent and 20 per cent of the original values. The final partial pressures of the residual gases as measured on the mass spectrometer were as follows (in units of 10-10 torr):
IOOO "}N2 ADMITTED TO 5microns. 9OO SUBLIMATION PUMPED (No 800 ION PUMP)TO 6'l(~6Torr 700 (a) 600 500 400 A~UNI:)~c~E/'~rlVE ! ~ L 300 200 I OO O , , I "30 " " ~404244
(~ 85~/oTi/IS°/omO ALLOY 2 ~RELATIVE .[ABUNDANCE |
FILAMENTCYCLED AT
lH20
. . . . .
45omR.~% I ......
I ,
2 12 14 16 IB 2022242628303234 3638404244 Figure 4. Residual gases after pump-down: (a) After sublimation pumping
(b) After sublimation pumping with getter-ion pumping
$ W Newton and ,I O'Neill: Mass spectrometric studies of a vacuum system Water (40 units), hydrogen (28), methane (5), nitrogen with carbon monoxide (5), argon (5), carbon dioxide (2), oxygen (<0.76). A low temperature bake under vacuum was now carried out at 140°C for two days, with the MS10 head at 300°C. N o marked changes in the composition of the residual gases were found, the reduction in height of the water vapour lines being only by 10 per cent. The composition of residual gases was also found to be very similar when using the other two types of sublimation pump filament already described. 5. P u m p - d o w n
curves
The pumping speed of an ion pump varies from gas to gas. The highest quoted speed is that for hydrogen with a speed of 270 per cent of that for air, and the lowest is argon with a speed of 6 per cent of that for air. The absorption and desorption rates of the internal surfaces of the system will also vary from gas to gas. Hence, different clean-up rates can be expected for different gases. Figure 5 shows a typical pump-down curve for the system after filling with dry nitrogen at atmospheric pressure for 1 hour. Initial pumping was by sorption pump to 5 × 10-3 torr, then by getter-ion pump only. Hydrogen, water and methane all gave a rapid initial drop of pressure followed by a slower rate of fall. Neon, argon and nitrogen however show a more constant rate of fall. Although the pump speed for neon is only a few per cent of that for hydrogen, the pressure of neon fell more rapidly than that of hydrogen. The rare gases are not absorbed by metals under pure]y thermal conditions, so the only available neon is that in the volume of the system, together with any which might have been absorbed in the pump electrodes during operation. In the case of hydrogen, however, there
is considerable absorption by the walls of the entire system. This gas will be slowly evolved, to give the effect of a virtual hydrogen leak. Hydrogen may also be evolved from the titanium cathode during operation. The high pressure of neon is rather unexpected. It did not occur in the earlier tests, when nitrogen filling was always used, but it has occurred consistently since the system has been regularly filled with air, and the pump allowed to operate against air leaks. Figure 6 shows the pump-down curves for the major gases when a sublimation pump and ion pump are used together. The system was initially filled v~"h dry air at atmospheric pressure for 1 hour. The different constituents now all showed more similar rates of clean-up. Again, hydrogen and water vapour showed the highest pressures at all stages from 10-5 torr downwards, and argon and oxygen were only present in very small amounts. There was a slight irregularity in the argon curve. This effect has been noticed on other occasions, and consists of a temporary increase of the argon pressure of up to 100 per cent lasting for a period of up to 30 minutes. N o effect on the total pressure was noticed as the argon pressure was only a small percentage of the total. This test was repeated with the system initially filled with oxygen instead of air. Very similar results were again obtained, the amount of oxygen still being very low, as shown in the dotted curve of Figure 6. When the total pressure had fallen to 10-5 torr the oxygen pressure was already reduced to 1 per cent of the total residual gas present. No attempts were made to measure the speed of the sublimation pump as the conductance of the tubing between the two pumps is only 300 litres/second, much lower than the expected speed of the sublimation pump, about 1500 litres/second. However, the effect of the high speed of this type of pump is shown in Figure 7. The system was filled to atmospheric pressure with oxygen for one hour, and pumping was carried out by sorption
iO -6
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PRESSURE Tort
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i
l
3 2
o
s'o
LIO-"
300
TIME MINUTES Figure 5. Pump-down curves, getter-ion pump, dry nitrogen filling
io-IO
3 :2
io--n
0
200
0 TiME
600
i2oo
MINUTES
Figure 6. Pump-down curves, getter-ion pump with sublimation pump. Dry air filling 679
S HI Newton and J O'Neill: Mass spectrometric studies of a vacuum system ION "-PUMP ONLY.
!ON PUMP WITH
12 II IO 9
PRESSURE8
i016
~,
GETTER ION PUMP
Tor.__.Er
7 6
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5 4 3 2
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SUBLIMATION "",~
N¢ . . . . . . .
"i': ......
.CON'rlNOUS
DUR.IN(; t~MINUTE
o..,
O I 234 567 8910 TIME IN MINUTES
9 8
iO-7
7' PRESSURE 6 Tort x IO -I°
iO-~
PAR T IAL
5 4 ¸ 3 2 I O
IN
O-9
5 4 3
I
O
I I I I I I I I I I I io-'O 15 3 0 4 5 60 75 9 0 IO5 120135 150165 180
TIME
IN MINUTES
Figure 7. Pump-down curves after filling with atmospheric pressure of oxygen
pump followed by getter-ion pump until the pressure was only falling slowly. After 130 minutes of getter-ion pumping, the sublimation pump was turned on, cycling for 1 minute "on", 3 minutes "off". The pressure of all the residual gases, including the noble gases argon and neon, now showed a rapid fall in pressure. From this it appears that a sublimation pump can, in some circumstances, cause rapid pumping of the noble gases as well as of gases such as hydrogen and nitrogen.
6. Gases evolved from sublimation pump filament The filaments were thoroughly cleaned before use with organic solvents followed by ultrasonic vibration under deionised water. In the initial degassing the filaments were heated in vacuum by ac, the current being increased in steps until the filament was at the normal operating temperature for pumping. At each stage almost all the gas evolved was hydrogen. Other gases present were water vapour, methane, neon, nitrogen, argon, carbon dioxide and ethane, in descending order of quantities.
680
~
. CO=
O
PRESSURE
Torr
~ I
2 34
...........
5 6 7 8910
TIME IN MINUTES
Figure 8. Partial pressures during pumping cycle
During cycling of the sublimation pump, the gas pressure rose as the filament temperature increased and fell again rapidly when the filament was switched off, and continued to fall for several minutes. Figure 8 shows the rise in pressure of the main gases during cycling. The filament was switched on for the period from 0 to 1 minute, and was off from 1 minute to 10 minutes. Hydrogen showed a particularly large peak during cycling, but both the mass 17 and mass 18 lines of water vapour showed no variation.
Conclusions A sublimation pump operated alone in a stainless steel system was effective in reducing the pressure from 5 × 10-3 tort to 6 x 10-6 tOll'. This pressure reduction was caused by pumping of the active gases, the inert gases not being pumped. A getter-ion pump used under similar conditions pumped all gases, the main residual being hydrogen. The use of a sublimation pump in parallel with the getter-ion pump now reduced the pressure of all the gases present, the greatest reduction being with hydrogen, but inert gases were also pumped. The residual gases present at pressures of lower than 10-5 torr were dependent mainly on the history of the surfaces of the system, and the type of pump used, the filling gas having relatively little effect. Acknowledgement
The authors wish to thank Messrs Ferranti Limited, Wythenshawe, Manchester for permission to publish this paper.
Reference 1 G M McCracken,
Vacuum, 15 (9), Sept 1965, 433.
The operation of a titanium sublimation pump with various types of filament is described. The partial pressures of residual gases in a stainless steel system were investigated during pump-down and also at the base pressures reached. The main residual gas after getter-ion pumping was hydrogen. The use of a sublimation pump in parallel with the getter-ion pump reduced the pressures of all gases, the main reduction being with hydrogen. A sublimation pump used alone did not pump the inert gases, but rapid pumping of all gases present was observed when both pumps were used together. The composition of residual gases after the pressure had fallen to 10-s torr during a normal pump-down was not greatly affected by the gas used for filling. At this pressure the composition was determined mainly by the gases evolved from the walls of the system. 1. Introduction Sublimation pumps are being increasingly used in conjunction with getter-ion pumps for the production of ultra-high vacua, and for the rapid evacuation of large volumes. They have a high pumping speed, and are simple to use. Their operation depends on the gettering action of a freshly deposited layer of titanium, and no ionising voltages nor magnetic fields are applied. The following study has been made by mass spectrometer of the residual gases in a system with both kinds of pump. 2. Description of system The system was constructed as in Figure 1, and consisted of a stainless steel cylinder of average diameter 6 inches and 50 inches long. It was fitted with a nude ion gauge (Bayard-Alpert type), Pirani gauge, viewing port and mass spectrometer (AEI SUBLIMATION PUMP TERMINALS
SUBLIMATION PUMP
CTROMETER NUDE ION GAI GAUGE NE--
VIEWING PORT
--~.~
..... rZ~_ 3
~
'~--'~:'VALVES
VOLUME OF SYSTEM 25 LITRES MATERIAL STAINLESSSTEEL
Figure 1. Vacuum system with sublimation pump
type MS10). At one end was a getter-ion pump (Ferranti FJD80) and at the other end was a sublimation pump (Ferranti FED10). Water cooling was applied to the regions surrounding the sublimation pump and the ionisation gauge. Before the series of measurements described in this paper the pumps had been operated whilst the whole system was baked at 200°C for three days. 3. S u b l i m a t i o n p u m p The sublimation pump had three filaments of an alloy containing 85 per cent titanium and 15 per cent molybdenum, 2.1 mm in diameter and 130 mm long, as described by McCracken 2. The filaments were used sequentially, being heated by ac to a temperature sufficient to evaporate the titanium on to the adjacent walls of the system, thereby providing the pumping action. Typical lives of the filaments for continuous operation ranged from 7 hours at 55 amps operating current to 15 hours at 45 amps. However, the filaments were normally cycled when the vacuum was better than 10-6 tort, so giving an increased overall life. A 25 per cent cycle, 1 minute "on", 3 minutes "off", was used down to 10- s torr, and 10 per cent or less when the pressure was still lower. Constant current operation was used. The voltage to be applied increased during life by about 60 per cent, so giving an indication of the remaining life of the filament. Pumping action was obtained throughout life, until bu'.T~-out occurred. At a running current of 55 amps, the average rate of deposition of titanium was approximately 3.4 micrograms per sq cm per minute. Figure 2 shows the appearance of a pump with filaments after completion of life test. The straightest filament is of the Ti/Mo alloy, and shows a marked crystal growth with offsetting of the crystals. Several grooves are also to be seen running circumferentiaUy round the wire at the crystal boundaries, indicating possibly some form of chemical attack by gas at these positions. At an earlier stage of life, the filament was bowed in a similar manner to the other filaments shown, but it had pulled straight again before burn-out occurred. 40 per cent of the available titanium had been used during life. The other two filaments consisted of a core of three strands of titanium wire, with tungsten overwinding. The alloy filament has lost
Vacuum/volume 16/number 12. Pergamon Press LtdlPrinted in Great Britain
677
S W Newton and d O'Neill: Mass spectrometric studies of a vacuum system 12 II
IO 9 8
(o)
7 6 5
(])GETTER ION PUMP
RELATIVE4 A~UNDAN~ I
O
2
:,1 I
12 1~I 16 18
12 202'22426 30323'43038404244 m/e
Figure 2. Sublimation pump filaments after completion of life test titanium uniformly from the entire length, but the other type has lost material only from the region close to the position of burn-out. This results in a short life and inefficient pumping due to the restricted area of the film of condensed titanium. A third type of filament has been used in a few tests only, having a tungsten core with two overwinding wires of titanium and one of molybdenum. This type did not always burn out when the titanium had been completely evaporated, so continuity of the filament is not a positive indication of satisfactory pumping action in a system without vacuum gauges. 4. R e s i d u a l gases after p u m p - d o w n
The mass spectra obtained on different days under similar conditions did not all show identical results, but the general trend was always maintained. In the first experiment, the system was evacuated to 10-8 tort, the ion pump switched off and dry nitrogen from boiledoff liquid nitrogen, admitted to a pressure of 5.2 x l0 -3 torr. The ion pump was now operated until the pressure fell to 5 x 10- s torr. Figure 3 shows the spectrum obtained on the MS10. Hydrogen (mass 2) showed the most prominent line, with water (masses 17 and 18) and methane (masses 12-13-1415-16) also prominent. Smaller amounts of nitrogen and carbon monoxide (mass 28), argon (mass 40) and carbon dioxide (mass 44) were present. The sublimation pump was now operated until the pressure was reduced to 5 × 10-9 tort. The most noticeable change in the mass spectrum (Figure 3b), was the reduction in hydrogen content from 12.3 units to 0.8 units, but all lines were somewhat reduced. The effect of using a sublimation pump without a getter-ion pump is shown in Figure 4a. Normally a sublimation pump is used in parallel with an ion pump. Nitrogen was admitted to a pressure of 5 × 10-3 ton'. On operating the sublimation pump, the pressure dropped rapidly to 1.6 × 10-s ton. within 1 minute, and then fell slowly to 6 × 10-6 torr in 45 minutes. The main residual gas now was the rare gas argon, the calculated line height being 1600 units, with methane, nitrogen, neon, carbon dioxide and ethane also present. Hydrogen and water vapour were only present in very small quantities. A similar test, in which the system was filled with dry air at atmospheric pressure instead of nitrogen gave a very similar spectrum, except that the heights of the neon lines were considerably increased. In both cases the amount of oxygen present was below the sensitivity limit of the mass spectrometer, 0.02 units, corresponding to a pressure of 0.76 × 10-*o ton'. 678
(b)
~
-,~ GETTER ION PUMP WITH SUBLIMATIONPUMP
t I
,Ill
,
2 ,2,4,6,8
..,
. . . . .
I
.,
2 0 2 2 24 2 6 2 s 3 0 3 2 3 4 3 6 , 3 s 4 0 , 2
m/¢
44
Figure 3. Residual gases after pump-down:
(a) After getter-ion pumping (b) After getter-ion pumping with sublimation pumping The getter-ion pump was now operated until the base pressure of 1.8 × 10-9 ton. was reached as measured on the ion gauge. The most marked changes in the mass spectrum occurred with the rare gases, argon and neon. These pressures fell to less than 0.01 per cent of the value with sublimation pump alone, but significant reductions were noticed with all gases. Hydrogen and water vapour had already been pumped by the sublimation pump, for the pressures in these cases were only reduced to 7 per cent and 20 per cent of the original values. The final partial pressures of the residual gases as measured on the mass spectrometer were as follows (in units of 10-10 torr):
IOOO "}N2 ADMITTED TO 5microns. 9OO SUBLIMATION PUMPED (No 800 ION PUMP)TO 6'l(~6Torr 700 (a) 600 500 400 A~UNI:)~c~E/'~rlVE ! ~ L 300 200 I OO O , , I "30 " " ~404244
(~ 85~/oTi/IS°/omO ALLOY 2 ~RELATIVE .[ABUNDANCE |
FILAMENTCYCLED AT
lH20
. . . . .
45omR.~% I ......
I ,
2 12 14 16 IB 2022242628303234 3638404244 Figure 4. Residual gases after pump-down: (a) After sublimation pumping
(b) After sublimation pumping with getter-ion pumping
$ W Newton and ,I O'Neill: Mass spectrometric studies of a vacuum system Water (40 units), hydrogen (28), methane (5), nitrogen with carbon monoxide (5), argon (5), carbon dioxide (2), oxygen (<0.76). A low temperature bake under vacuum was now carried out at 140°C for two days, with the MS10 head at 300°C. N o marked changes in the composition of the residual gases were found, the reduction in height of the water vapour lines being only by 10 per cent. The composition of residual gases was also found to be very similar when using the other two types of sublimation pump filament already described. 5. P u m p - d o w n
curves
The pumping speed of an ion pump varies from gas to gas. The highest quoted speed is that for hydrogen with a speed of 270 per cent of that for air, and the lowest is argon with a speed of 6 per cent of that for air. The absorption and desorption rates of the internal surfaces of the system will also vary from gas to gas. Hence, different clean-up rates can be expected for different gases. Figure 5 shows a typical pump-down curve for the system after filling with dry nitrogen at atmospheric pressure for 1 hour. Initial pumping was by sorption pump to 5 × 10-3 torr, then by getter-ion pump only. Hydrogen, water and methane all gave a rapid initial drop of pressure followed by a slower rate of fall. Neon, argon and nitrogen however show a more constant rate of fall. Although the pump speed for neon is only a few per cent of that for hydrogen, the pressure of neon fell more rapidly than that of hydrogen. The rare gases are not absorbed by metals under pure]y thermal conditions, so the only available neon is that in the volume of the system, together with any which might have been absorbed in the pump electrodes during operation. In the case of hydrogen, however, there
is considerable absorption by the walls of the entire system. This gas will be slowly evolved, to give the effect of a virtual hydrogen leak. Hydrogen may also be evolved from the titanium cathode during operation. The high pressure of neon is rather unexpected. It did not occur in the earlier tests, when nitrogen filling was always used, but it has occurred consistently since the system has been regularly filled with air, and the pump allowed to operate against air leaks. Figure 6 shows the pump-down curves for the major gases when a sublimation pump and ion pump are used together. The system was initially filled v~"h dry air at atmospheric pressure for 1 hour. The different constituents now all showed more similar rates of clean-up. Again, hydrogen and water vapour showed the highest pressures at all stages from 10-5 torr downwards, and argon and oxygen were only present in very small amounts. There was a slight irregularity in the argon curve. This effect has been noticed on other occasions, and consists of a temporary increase of the argon pressure of up to 100 per cent lasting for a period of up to 30 minutes. N o effect on the total pressure was noticed as the argon pressure was only a small percentage of the total. This test was repeated with the system initially filled with oxygen instead of air. Very similar results were again obtained, the amount of oxygen still being very low, as shown in the dotted curve of Figure 6. When the total pressure had fallen to 10-5 torr the oxygen pressure was already reduced to 1 per cent of the total residual gas present. No attempts were made to measure the speed of the sublimation pump as the conductance of the tubing between the two pumps is only 300 litres/second, much lower than the expected speed of the sublimation pump, about 1500 litres/second. However, the effect of the high speed of this type of pump is shown in Figure 7. The system was filled to atmospheric pressure with oxygen for one hour, and pumping was carried out by sorption
iO -6
iO-6
i0-~
10-'7
iO-8
iO--a PART I A L
PARTIAL PRESSURE Tort
PRESSURE Tort
10-9
iO-~ ~. )a
)-IO
~ 0 2 WITH 02 FILLING " \ ~
i
l
3 2
o
s'o
LIO-"
300
TIME MINUTES Figure 5. Pump-down curves, getter-ion pump, dry nitrogen filling
io-IO
3 :2
io--n
0
200
0 TiME
600
i2oo
MINUTES
Figure 6. Pump-down curves, getter-ion pump with sublimation pump. Dry air filling 679
S HI Newton and J O'Neill: Mass spectrometric studies of a vacuum system ION "-PUMP ONLY.
!ON PUMP WITH
12 II IO 9
PRESSURE8
i016
~,
GETTER ION PUMP
Tor.__.Er
7 6
l'; I """
xiO-9
5 4 3 2
l \!
SUBLIMATION "",~
N¢ . . . . . . .
"i': ......
.CON'rlNOUS
DUR.IN(; t~MINUTE
o..,
O I 234 567 8910 TIME IN MINUTES
9 8
iO-7
7' PRESSURE 6 Tort x IO -I°
iO-~
PAR T IAL
5 4 ¸ 3 2 I O
IN
O-9
5 4 3
I
O
I I I I I I I I I I I io-'O 15 3 0 4 5 60 75 9 0 IO5 120135 150165 180
TIME
IN MINUTES
Figure 7. Pump-down curves after filling with atmospheric pressure of oxygen
pump followed by getter-ion pump until the pressure was only falling slowly. After 130 minutes of getter-ion pumping, the sublimation pump was turned on, cycling for 1 minute "on", 3 minutes "off". The pressure of all the residual gases, including the noble gases argon and neon, now showed a rapid fall in pressure. From this it appears that a sublimation pump can, in some circumstances, cause rapid pumping of the noble gases as well as of gases such as hydrogen and nitrogen.
6. Gases evolved from sublimation pump filament The filaments were thoroughly cleaned before use with organic solvents followed by ultrasonic vibration under deionised water. In the initial degassing the filaments were heated in vacuum by ac, the current being increased in steps until the filament was at the normal operating temperature for pumping. At each stage almost all the gas evolved was hydrogen. Other gases present were water vapour, methane, neon, nitrogen, argon, carbon dioxide and ethane, in descending order of quantities.
680
~
. CO=
O
PRESSURE
Torr
~ I
2 34
...........
5 6 7 8910
TIME IN MINUTES
Figure 8. Partial pressures during pumping cycle
During cycling of the sublimation pump, the gas pressure rose as the filament temperature increased and fell again rapidly when the filament was switched off, and continued to fall for several minutes. Figure 8 shows the rise in pressure of the main gases during cycling. The filament was switched on for the period from 0 to 1 minute, and was off from 1 minute to 10 minutes. Hydrogen showed a particularly large peak during cycling, but both the mass 17 and mass 18 lines of water vapour showed no variation.
Conclusions A sublimation pump operated alone in a stainless steel system was effective in reducing the pressure from 5 × 10-3 tort to 6 x 10-6 tOll'. This pressure reduction was caused by pumping of the active gases, the inert gases not being pumped. A getter-ion pump used under similar conditions pumped all gases, the main residual being hydrogen. The use of a sublimation pump in parallel with the getter-ion pump now reduced the pressure of all the gases present, the greatest reduction being with hydrogen, but inert gases were also pumped. The residual gases present at pressures of lower than 10-5 torr were dependent mainly on the history of the surfaces of the system, and the type of pump used, the filling gas having relatively little effect. Acknowledgement
The authors wish to thank Messrs Ferranti Limited, Wythenshawe, Manchester for permission to publish this paper.
Reference 1 G M McCracken,
Vacuum, 15 (9), Sept 1965, 433.